A time-of-flight or TOF mass spectrometer can be used to determine the mass of ions torn from a sample by measuring the time of flight of the ions from a determined position, real or virtual, of the ion source, to their impact on a detector, through an analysis chamber. The time of flight of an ion through an electrostatic field is proportional to the square root of the mass-to-charge ratio of this ion for a given kinetic energy. The mass resolution of a time-of-flight spectrometer depends, in addition to the accuracy with which the instants of departure and of impact of the ions can be measured, on the energy dispersion of the ions; as it happens, ions with the same mass-to-charge ratio but with different energy-to-charge ratios exhibit different times of flight from the ion source to the detector.
One known method that can be used to eliminate, at least as a first approximation, the dependency of the time of flight of an ion on its energy, and thus enhance the mass resolution of a time-of-flight spectrometer, is to incorporate in the analysis chamber of the mass spectrometer a device of ion mirror type. This method was proposed for the first time by Alikhanov, and implemented by Mamyrin. Reference can be made to the corresponding respective articles: Alikhanov, Soviet Physics Journal of Experimental and Theoretical Physics (JETP), 4 (1956) 452 and Mamyrin et al., Soviet Phys. JETP, 37 (1973) 45.
The ions with the greatest energy penetrate more deeply into the electrostatic field generated by the ion mirror, the path that they travel and their time of flight in the analysis chamber are thus longer than for the ions with weaker energy. Consequently, the extension of the time of flight of the ions with the greatest energy compared to the ions with the least energy within the electrostatic field generated by the ion mirror compensates for the fact that the time of flight is shorter for the ions with the greatest energy, in the area of the analysis chamber located outside the field generated by the mirror. Thus, the total time of flight within the analysis chamber is made independent of the energy of the ions. The mass analyzers—that is to say notably the mass spectrometers and atom probes—equipped with electrostatic mirrors are commonly referred to as reflectron mass analyzers.
The ion mirrors used in the reflectron time-of-flight mass analyzers typically incorporate delaying electrostatic fields that are uniform or uniform piecewise, that is to say uniform in determined spatial regions. The ion mirrors commonly consist of a main electrode, of a particular geometry and excited by an electrical potential, and a gate electrode of a similar geometry and excited by a different electrical potential. The electrostatic field generated by these electrodes is contained in the space separating these electrodes, and its characteristics can, for example, be adjusted according to the excitation potentials.
Different types of mass spectrometers rely on varying ion emission methods such as field desorption, laser desorption or secondary ion emission, which are in themselves known from the state of the art. One characteristic of the ion beams resulting from these techniques is a strong angular dispersion of the emitted ions, that can have values as high as around 90° or more. It is important not to restrict ion beams with wide angular dispersion, in order to maximize the sensitivity of the mass spectrometer. Furthermore, the analysis of ions emitted with a wide angular dispersion can also be of major interest, in certain applications such as, for example, atom probes, also called atom probe microscopes, in which an increase in the angular acceptance is synonymous with a widening of the field of vision of the microscope, given that different emission angles correspond to different positions on the surface of the sample from which the ions are torn.
It should be noted that all the ion emission methods, and more particularly field desorption, are characterized by significant energy dispersions; it is thus particularly indicated to use ion mirrors in order to improve the performance levels of the time-of-flight mass analysis devices.
The ion mirrors of the conventional reflectrons with piecewise uniform electrostatic field cannot accept an angular dispersion of the ion beam greater than approximately 10°. In order to make it possible to record the ion signals with detectors of reasonable dimensions, while accepting strong angular dispersions, mirrors with curved geometry were proposed in the article by Vialle et al., Rev. Sci. Instrum., 68 (1997) 2312. A reflectron with curved geometry is proposed in the international patent application WO2006/120428. This type of reflectron produces a transformation of the ion beam diverging from a sample of small size into a substantially parallel beam which can be admitted by a detector of reasonable dimensions. The plane of the detector is substantially perpendicular to the ion beam, in order to avoid increasing the dimensions of the detector, which would otherwise be unavoidable. In addition to its spatial focusing properties, and the focusing in terms of time of flight as a function of the energy of the ions, such a device has spatial focusing properties as a function of the energy of the ions, and can thus be used to obtain images of a sample that are spatially resolved, in atom probe microscopes.
Such a reflectron does, however, have some drawbacks. On the one hand, the angular acceptance of such a device cannot exceed 90° for reasons simply linked to the geometry of the device. The angular acceptance of such a reflectron is also reduced by an essential inclination relative to the plane of the detector, of the surface on which the focus in terms of time of flight as a function of energy is produced. Another drawback with this type of reflectron is linked to the fact that the intersection between most of the trajectories of the ions and the direction normal to the input gate electrode of the reflectron mirror is produced according to fairly open angles, which considerably increases the dispersion of the ions at the level of the local electrical field non-uniformities generated by the gate.
To sum up, the use of curved field mirrors improves the angular acceptance with mass spectrometers or reflectron atom probe microscopes. However, the reflectrons known from the state of the art do not give these devices a sufficient angular acceptance, and offer a certain number of other drawbacks.